Feed system for an ion thruster

ABSTRACT

A system for delivering vaporized mercury to an electron bombardment ion engine characterized by a source of liquid mercury, pressurized by Freon vapor for force-feeding liquid mercury to a mercury vaporizer operated at a temperature and a pressure sufficient to deliver vaporized mercury at a constant, low-rate flow to an associated ion engine.

United States Patent Inventors Appl. No. Filed Patented July 13, 1971 Paine. T. 0. 1.

ReferencesCited Administrator of the National Aeronautics UNITED STATES PATENTS and Space Admmlstratlon, with respect to 2,844,938 7/1958 Longwell 2 875 589 3/1959 H Tommy D. Masek 3788 Cloud Ave. om La Crescema Cam.- 2,924,359 2/1960 Beremand 760114 I 3,156,090 9/1961 Kaufman i 5 1968 3,359,733 12/1967 Forbes 3,443,383 5/1969 King Primary ,Examiner-Samuel Feinberg Attorneys-.1. H. Warden, M. F. Mott and G. T. McCoy FEED SYSTEM FOR AN ION THRUSTER 1 Claim, 3 Drawing Figs.

US. Cl

Int. Cl

.............................................. 60/202, ABSTRACT: A system for delivering vaporized mercury to an 60/39.48 electron bombardment ion engine characterized by a source F03h of liquid mercury, pressurized by Freon vapor for force-feed- H05h 1/00 ing liquid mercury to a mercury vaporizer operated at a tem- Field of Search 60/202, perature and a pressure sufficient to deliver vaporized mercu- 39.48, 36, 37, 50 ry at a constant, low-rate flow to an associated ion engine.

CONTROL CIRCUIT MAIN POWER SUPPLY PATENTED JUL13IHH 7 3,591,967

CONTROL CIRCUIT MAIN POWER SUPPLY TOMMY D. MASEK wvmrm ATTORNEY FEED SYSTEM FOR AN ION TI-IRUSTER ORIGIN OF THE INVENTION The invention described herein was made in the performance of work under a NASA contract and is subject to the provisions of Section 305 of the National Aeronautics and Space Act of i958, Public Law 85-568 (72 stat. 435; 42 U.S.C. 2457).

BACKGROUND OF THE INVENTION 1 Field of the Invention The invention relates to propellant feed systems and more particularly to a relatively low-flow-rate delivery system for delivering vaporized mercury to an electron bombardment ion engine for imparting ion propulsion to an associated spacecraft.

Ion propulsion involves the ionization of atoms of a propellant followed by an acceleration of the resulting ions. Ionization normally is achieved in a chamber of an ion engine wherein ionization of a vaporized propellant takes place in the presence of electric and magnetic fields. As is well known, to achieve ionization electrons are injected into the engine to bombard and thus ionize a vaporized propellant, usually a metal, to establish a plasma which is electrically charged and which is capable of being accelerated electrically from the engine to acquire the necessary thrust.

In order to conduct propellant to an ion engine, it is necessary to establish a suitable storage and feed system. Such systems necessarily include a provision for vaporizing a liquid propellant and then delivering the vaporized propellant at a predetermined rate of flow. In practice, it has been found that a flow rate of about 1 cc. (cubic centimeter) per hour is adequate for a 2.5 kw. (kilowatt) thruster. However, changing power requirements will initiate changes in the required flow rates.

2. Description of the Prior Art Electron bombardment ion engines often require vaporized mercury, delivered at relatively low flow rates. As a practical matter, the flow rates frequently required are in the neighborhood of 1 cc. per hour. As the ion engine is particularly suited for use in outer space, the environment in which the engine is to be used is one of zero gravity. Therefore, various attempts have been made effectively to deliver liquid mercury at suitable rates in a zero gravity environment. The prior art systems frequently require relatively complex mechanical devices as well as electromechanical devices of a type electrically actuatable for positioning and discharging the given liquid propellants. Where cryogenic materials such as nitrogen and helium are employed as sources of pressurizing gases for pumping the liquid propellant materials, systems of a rather complex nature requiring a relatively large number of moving parts presently are found to exist. Where gases such as nitrogen or helium are employed, large gas storage volumes involving significant weight are required. Furthermore, flow rate control of gaseous pressurization systems has been found to be difficult because of the low flow rates required.

OBJECTS AND SUMMARY OF THE INVENTION This invention overcomes many of the aforementioned difficulties through the use of a novel system which includes a low-pressure Freon pressurizer for pumping a high-density liquid mercury from a source to a vaporizer which converts liquid mercury to a vapor and delivers the vapor at a desired rate.

Accordingly, an object of the invention is to provide a simplified and improved propellant feed system.

Another object is to provide a simplified lightweight feed system for delivering a propellant in a zero gravity environment.

Another object is to provide an economic and simplified feed system for delivering vaporized mercury to an ele tron bombardment ion engine.

Another object is to provide a feed system for delivering vaporized mercury to an electron bombardment ion engine employing a reduced number of movable components.

Another object is to provide a pressurized feed system for delivering liquid mercury under pressure to a vaporizer and converting the liquid mercury to vaporized mercury while employing a reduced number of components through the utilization of pressures of vaporization.

These together with other objects and advantages will become more readily apparent by reference to the following description and claims in light of the accompanying drawing.

DESCRIPTION OF THE DRAWING FIG. I is a diagrammatic view of a feed system embodying the principles of the instant invention for delivering vaporized mercury at a predetermined rate to a pair of ion engines.

FIG. 2 comprises a partially sectioned view of the mercury reservoir shown in FIG. 1.

FIG. 3 comprises a partially sectioned view of the vaporizer employed in the system shown in FIG. 1.

DESCRIPTION OF THE INVENTION Turning now to FIG. I, a system embodying the principles of the present invention is therein illustrated in association with well-known ion engines 10. These engines normally are mounted by suitable mans on a spacecraft, not shown, so as to provide the propulsion required for a given mission having predetermined mission requirements. The craft is provided with a suitable control circuit section 12 which houses various control circuits for the craft, including those for dictating the functions of the components of the herein-described propellant feed system. In addition to the control circuits 12, there is a main power circuit 14 which is employed for supplying the required electrical energy to the system components. While not shown in the drawing, the main power, or electrical energy, can be provided by any suitable means such as by solar panels, nuclear reactors, chemical or any other voltage sources, since the particular manner in which the power is supplied to the system embodying the principles of the present invention is deemed to be immaterial for the purposes of describing and understanding the invention.

The propellant to be delivered by the feed system includes liquid mercury which is delivered from or to a reservoir 20 through a discharge port 21, as a liquid, and then to a vaporizer 22. Here the propellant is vaporized and fed to the engines E0 in a vaporized and ionizable form whereupon it is ionized and utilized in the required manner.

The reservoir 20 is of a spherical configuration and includes two mated hemispheres. The hemispheres forming the reservoir 20 are connected at their bases to form a sphere. One of the hemispheres is provided with the discharge port 21, while within the reservoir 20 there is fixed a diaphragm or bladder 26 which in operation serves to force the propellant through the port 21. The bladder or diaphragm 26 is fabricated to assume a hemispheric configuration and can be operatively seated in either half of the reservoir 20 against the surface of the hemispheres. A flexible impermeable material such as Neoprene has been found to be particularly suited for fabricating the diaphragm 26 as it is substantially impenneable to liquid mercury. The bladder is anchored only at its peripheral edge by suitable means including an interlocking chime 28 formed between the bases of the mated hemispheres of the reservoir 20.

It is intended that the reservoir 20 be substantially filled with the propellant or liquid mercury prior to the launching of the spacecraft. Therefore, the bladder, as shown in FIG. 2, is disposed in a configuration wherein the liquid mercury propellant is permitted substantially to fill the reservoir 20. However, the bladder is operatively to be displaced in a direction extending toward discharge port 21 thus to displace the mercury from within the reservoir during the operation of the associated spacecraft.

In order to prevent a trapping of the liquid mercury between the bladder and the surface of the adjacent hemisphere as the bladder is displaced to engage the surface at the termination of discharge, elongated ribs or ridges 30 are provided for the bladders surface. These ridges may be molded into the surface of the bladder or diaphragm during its fabrication. In any event, the ribs 30 are so aligned as to extend toward the discharge port 21 of the reservoir 20 when the propellant has been discharged from the hemisphere.

In order to actuate the diaphragm for pumping mercury through the discharge opening 21, a pressurizer including a source of gas including a sealed container 32 has been provided. The container is connected through a conduit 33 and a one-shot valve 34 to an inlet port 36 provided opposite the discharge port 21 of the reservoir 20. The valve 34 is of any suitable type which may include a solenoid operated actuator which, upon command, serves to drive the valve to its open position. Once the valve 34 has been actuated to an open position, it is not contemplated that it will be closed during the life of the spacecraft, even though it is feasible to provide for a selective actuation of the valve if desired. If desired, the gas source or container 32 may be fabricated as an integral part of the propellant reservoir 20. In such case the valve 34 would not be required.

The gas provided by the source is a vaporized material or medium which is stored as a liquid in the container 32. As is well known, most liquids will vaporize at some given temperature. Furthermore, the resulting vapor will establish an equilibrium within the enclosure in which it is confined. Therefore, in practice, the pressure of the gas delivered to the reservoir 20 will be the same as that existing within the container.

Once the valve 34 is opened, the container 32, conduit 33, and expanding portion of the reservoir 20 serve to establish a closed chamber wherein an equilibrium can be established.

Spacecraft frequently operated with an onboard temperature ranging between 60 and 180 F. It has been determined that a pressure ranging from about 5 p.s.i.a. (pounds per square inch absolute) to about 50 p.s.i.a. is desirable for pumping or discharging the propellant from the reservoir. Consequently, Freon, commercially available as Freon I13, has been found to be particularly suited for such use, since it can be stored as a liquid and yet provide the necessary vapor pressures for pumping the high-density mercury at the particular temperature ranges to be operatively encountered. Hence, the container 32 of the source serves to store Freon l 13 in a liquid state from which it is permitted to vaporize and pass through the one-shot valve 34 to enter the reservoir through the opening or inlet 36.

It is known that the vapor pressure of the liquid Freon tends to remain substantially constant, even though there is a slow increase in the confining volume, throughout the operation of the spacecraft, so long as the temperature of the craft does not radically vary. It is also known that the liquid Freon will be converted to a gas as required to fill the confining volume. An initial volume of liquid Freon of about 80 cc. will provide adequate Freon vapor for pumping a reservoir containing 200 pounds of mercury. Hence, the pressure of the vapor acting against the Neoprene diaphragm or bladder 26 serves to force the mercury from the propellant reservoir 20 at a substantially constant pressure throughout its operative life. As the mercury is dispelled from the source 20, it is permitted to pass through a selectively operable on-off valve 40, which also is controlled by the circuit 12. This valve functions to direct the fluid through associated conduits 42 to the vaporizer 22. In practice, it is preferred that the conduit 42 function as a manifold for interconnecting as many vaporizers 22 with the reservoir 20 as is deemed desirable to assure the proper operation of the craft.

Turning now to FIG. 3, there is shown in section one of the vaporizers 22 schematically depicted in FIG. 1. As the vaporizers 22 employed in the disclosed embodiment of the present invention are of a similar design, it is deemed sufficient, in the interest of brevity, to describe only one of them.

As the mercury emerges from the conduit 42, it is introduced under pressure into a storage chamber or compartment 44 of the vaporizer. The chamber 44 is defined by a plurality of opposing walls 46, which are fabricated in any suitable fashion to confine the liquid mercury. Preferably, the chamber 44 is defined by a cylindrical wall 46 having an end closure at one end and a transversely aligned porous plate 48 disposed at a position opposite the end closure. The pores of plate 48 serve as vapor discharge conduits through which vaporized mercury is discharged from the storage chamber 44.

The wall 46 preferably is formed of tantalum while the porous plate 48 is formed of porous tungsten to avoid the wetting" effects which have been present after extended use where other metals, such as stainless steel were employed. To facilitate assembly of the vaporizer, the tungsten plate 48 is secured by welding or by other suitable techniques, at one end of a tubular member 50. The member 50 is machined into a configuration which permits it to be seated within and secured to the cylindrical wall 46. Therefore, it will be appreciated that at the surface of the porous tungsten plate 48, adjacent the liquid mercury there is, in operation, established a liquidvaporinterface. The opposite side of the plate 48 serves as an exit surface for the vaporized mercury as it is discharged from chamber 44. The tubular member 50 further includes an axially aligned mercury vapor exit or chamber 51. The chamber 51 receives the vapor discharged by the plate 48 and conducts it away from the vaporizer. In practice, the vaporizer 22 also is fabricated utilizing suitable known welding techniques.

The plate 48 may, if desired, be formed as a disc having its porous openings extending therethrough so that vaporized mercury may escape while the liquid is retained within the storage chamber 44 in engagement with the plate s surface. As presently employed, the plate 48 is of a thickness of approximately six-hundreths of an inch, while the pores therein are of an average pore size of 8 microns, ranging between 2 and 24 microns, with a density of 70 percent of theoretical.

As hereinbefore mentioned, the operative pressure of the mercury is between 5 and 50 p.s.i.a. At pressures less than 5 p.s.i.a., the pressure of the mercury vapor at the liquid-vapor interface is sufficient to prevent adequate vaporization of the liquid mercury at the surface of the plate 48, within the chamber 44, since the mercury vapor, in effect, establishes a back pressure which is sufficient to force the liquid mercury away from the surface of the plate 48. At pressures greater than 50 p.s.i.a., the liquid mercury is forced through the pores of the plate and discharged as a liquid into the tubular chamber 51. Hence, vapor pressures applied by the Freon 1 l3 vapor are particularly suited to establish a pressure of 5 to 50 p.s.i.a. for the mercury as it is supplied to the vaporizer 22.

In order to achieve adequate vaporization of the mercury within chamber 44 of the vaporizer 22, it is necessary that the mercury be maintained at a given suitable temperature. Such temperatures are normally within a range extending between 200 C. and 300 C. In practice, it has been found that to achieve a I cc. per hour rate of flow at the given pressure from the aforedescribed plate 48, a temperature of approximately 270 C. is required. Since the spacecraft is operating at a temperature of something less than 200 C. it is deemed desirable to provide a heating system for heating the mercury as it is retained within the storage chamber 44. Heating of the mercury is achieved through a plurality of turns of a suitable resistance heating coil 52. The heating coil 52 includes coaxial nickel wire 54 sheathed within a tube 56 of known design.

The coil 52 is operatively connected with the main power source 14 through the control circuits 12. In order to achieve a desired control of the temperature, any suitable means can be employed. However, it has been found particularly desirable to employ a simple thermocouple 58 seated in a suitable well machined within the external surface of the tubular member 50. The thermocouple 58 is in practice connected with a sensor control, not shown, located within the control circuit 12. Consequently, when the thermocouple 58 senses a significant change in temperature, a signal is directed to the control circuit 12, whereupon a heating circuit is made or broken, as required, to the resistance heating coil 52. As a matter of convenience, a shield 60 has been provided to surround the heating coil in a protective manner.

Vapor discharged from the mercury vapor zone 51 is directed downstream to the isolator 24. Therefore, the vaporizer 22 is provided with a mounting flange 62 inciuding openings 64 formed therein for joining the vaporizer 22 to a mated coupling 66. The coupling 66 is welded or otherwise coupled to a conduit 68 which serves to deliver vapor from the vaporizer 22 to the downstream components such as the isolator 24.

Very briefly, the isolator comprises a tubular member which serves as means for preventing ionization of the mercury vapor as it is discharged downstream to the ion engine As presently employed, a flat -mesh screen has been included at the downstream end of the isolator 24 in order to provide therein a plasma boundary. It is through the utilization of isolators 24 that the various engines 10 are electrically is0- lated thus to assure that the mercury vapor is permitted to enter the ionization chamber of the engines in an un-ionized state.

As isolators are of well-known design, it is not deemed necessary to describe the presently employed isolators in detail. It is believed sufficient to understand that the isolators 24, as presently employed, maintain a suitable breakdown voltage and can be fabricated from numerous materials such as boron nitride, or a high-density alumina, available commercially as Lucalox. Split ring clamps, flanges and other means can be readily employed in mounting the isolator 24 between the vaporizers 22 and the engines 10. It should be understood that the mercury vapor being discharged from the discharge chamber 51 of the vaporizer 22 is conducted by conduit 68 through the isolator 24, and is injected into the ion chamber of the engine to be used as a propellant in a manner common to electron bombardment ion engines.

OPERATION It is believed that in view of the foregoing description, the operation of the device will be readily understood, and, it will only be briefly reviewed at this point.

The container 32 includes Freon 1l3 stored therein in a liquid state but which is permitted to vaporize for employment by the feed system. The Freon in its vapor state escapes from the container 32 through the conduit 33 and the opened oneshot valve 34 to the mercury reservoir 20. The valve 34, in practice, previously has been opened in response to a signal supplied by the control circuit 12. As the liquid Freon evaporates, a state of equilibrium is established within the container 32 and the conduit 33. However, the conduit 33 communicates with the one surface of the Neoprene bladder 26 through an opening 36. Therefore, pressures resulting from the vaporization of the liquid Freon is applied directly to the outermost wall of the bladder 26, which in turn applies a pressure to the liquid mercury retained within the reservoir 20. This pressure, as it is exerted against the bladder, serves to discharge the mercury through the opening 21 of the reservoir 20 into the adjacent conduit 41 and cause it to progress through the valve 40 to be dispersed by the manifold or conduit 42. The valve 40 also previously has been opened in response to a signal derived from the control circuit 12. As the liquid mercury is discharged from the reservoir 20 in response to a buckling or reversing displacement of the bladder 26, the liquid mercury is caused to be discharged through the conduits to the vaporizers 22.

The vaporizers 22, in turn, serve to receive the mercury in the storage zone or chamber 44. The pressure at which mercury is supplied to the chamber 44 is substantially that of the Freon vapor pressure maintained within the conduit 33. Since the liquid mercury enters the chamber 44 under a preselected pressure of 5 to 50 p.s,i.a., the mercury liquid-vapor interface occurs along the adjacent surface of the plate 48. The

vaporized mercu is then ermitted to be dischargpd through the pores of the p ate into t e mercury vapor exit c amber 1.

It is apparent that various flow rates are achievable by varying the temperature of the liquid in the chamber 44. However, in

order to achieve a desired rate of discharge of a vapor from the vaporizer 22 to deliver a flow of vapor at a flow rate of about 1 cc. per hour, the vaporizer is maintained at a temperature of approximately 270 C.

As the vaporized mercury is discharged from the vaporizer 22, it is conducted by the conduit 68 to the isolator 24 and thence to the associated ion chamber 10 at which point the mercury vapor is ionized and employed in a fashion well understood by those versed in the art of ion engines.

Although the invention has been herein shown and described in what is conceived to be the most practical and preferred embodiment, it is recognized that departures may be made therefrom within the scope of the invention, which is not to be limited to the details disclosed herein but is to be accorded the full scope of the claims so as to embrace any and all equivalent devices and apparatus.

What I claim is:

ll. A feed system for an ion thruster comprising:

a. means defining an hermetically sealed chamber including an inlet and an outlet orifice;

b. a flexible, impermeable diaphragm disposed between said inlet and said outlet orifice dividing said chamber into a first and a second hermetically sealed compartment;

c. a high-density liquid metal disposed within said first compartment substantially filling said chamber and adapted to be discharged through said outlet orifice;

d. a source of Freon gas under pressure coupled with said inlet orifice adapted to deliver to said second compartment a Freon gas at a pressure as great as the pressure of the high-density liquid metal;

e. a vaporizer operatively coupled with the outlet orifice of said chamber including a storage chamber defined by:

I. a cylindrical wall sealed at one end by a transverse wall including means defining therein an inlet orifice for receiving said high-density liquid metal, and

2 a transverse plate formed of porous tungsten having interstices sufficiently great for passing therethrough metal in a vapor state secured to said cylindrical wall and spaced from said end wall for confining said highdensity liquid metal therebetween;

. an electrically energizable heating means operatively. as-

sociated with said vaporizer for heating the high-density metal to a temperature sufficient for vaporizing the metal whereby a liquid vapor interface is established along a first face of said porous plate so that the metal in its vapor state is afforded discharge from said storage chamber through the plate;

g. sensing means coupled with the heating means for maintaining the temperature of said high-density liquid within a predetermined range; and h. means defining an exit chamber disposed adjacent to the second face of said plate and maintained at a pressure substantially lower than the pressure of said storage chamber. 

1. A feed system for an ion thruster comprising: a. means defining an hermetically sealed chamber including an inlet and an outlet orifice; b. a flexible, impermeable diaphragm disposed between said inlet and said outlet orifice dividing said chamber into a first and a second hermetically sealed compartment; c. a high-density liquid metal disposed within said first compartment substantially filling said chamber and adapted to be discharged through said outlet orifice; d. a source of Freon gas under pressure coupled with said inlet orifice adapted to deliver to said second compartment a Freon gas at a pressure as great as the pressure of the high-density liquid metal; e. a vaporizer operatively coupled with the outlet orifice of said chamber including a storage chamber defined by:
 1. a cylindrical wall sealed at one end by a transverse wall including means defining therEin an inlet orifice for receiving said high-density liquid metal, and
 2. a transverse plate formed of porous tungsten having interstices sufficiently great for passing therethrough metal in a vapor state secured to said cylindrical wall and spaced from said end wall for confining said high-density liquid metal therebetween; f. an electrically energizable heating means operatively associated with said vaporizer for heating the high-density metal to a temperature sufficient for vaporizing the metal whereby a liquid vapor interface is established along a first face of said porous plate so that the metal in its vapor state is afforded discharge from said storage chamber through the plate; g. sensing means coupled with the heating means for maintaining the temperature of said high-density liquid within a predetermined range; and h. means defining an exit chamber disposed adjacent to the second face of said plate and maintained at a pressure substantially lower than the pressure of said storage chamber.
 2. a transverse plate formed of porous tungsten having interstices sufficiently great for passing therethrough metal in a vapor state secured to said cylindrical wall and spaced from said end wall for confining said high-density liquid metal therebetween; f. an electrically energizable heating means operatively associated with said vaporizer for heating the high-density metal to a temperature sufficient for vaporizing the metal whereby a liquid vapor interface is established along a first face of said porous plate so that the metal in its vapor state is afforded discharge from said storage chamber through the plate; g. sensing means coupled with the heating means for maintaining the temperature of said high-density liquid within a predetermined range; and h. means defining an exit chamber disposed adjacent to the second face of said plate and maintained at a pressure substantially lower than the pressure of said storage chamber. 